Attractive materials for use as cathode materials for lithium ion secondary batteries include LiCoO2, LiNiO2, and LiMn2O4. Unlike LiCoO2 and LiNiO2, the LiMn2O4 spinel compounds are believed to be overcharge safer and are desirable cathode materials for that reason. Nevertheless, although cycling over the full capacity range for pure LiMn2O4 can be done safely, the specific capacity of LiMn2O4 is low. Specifically, the theoretical capacity of LiMn2O4 is only 148 mA·hr/g and typically no more than about 115-120 mA·hr/g can be obtained with good cycleability. LiMn2O4 can contain excess lithium on the 16d manganese sites and can be written as Li1+xMn2-xO4 (0≦x≦0.33). Use of the formula LiMn2O4 herein is understood to denote Li1+xMn2-xO4 as well.
The orthorhombic LiMnO2 and the tetragonally distorted spinel Li2Mn2O4 have the potential for larger capacities than those obtained with the LiMn2O4 spinel. However, cycling over the full capacity range for LiMnO2 and Li2Mn2O4 results in a rapid capacity fade. Layered LiMnO2 quickly converts to a spinel form upon cycling which also results in a capacity fade.
Various attempts have been made to either improve the specific capacity or safety of the lithium metal oxides used in secondary lithium batteries by doping these lithium metal oxides with other cations. For example, U.S. Pat. No. 6,214,493 to Bruce et al. relates to stabilized layered LiMnO2 using cobalt (Co) as a dopant material. Stabilization has been recorded with as little as 15 percent cobalt substitution. In another example, U.S. Pat. No. 5,370,949 to Davidson et al. proposes that introducing chromium cations into LiMnO2 can produce a tetragonally distorted spinel type of structure which is air stable and has good reversibility on cycling in lithium cells.
Li2MnO2 compounds have also been considered as electrode materials. U.S. Pat. No. 4,980,251 to Thackeray proposes that Li2MnO2 can be formed having a theoretical capacity of 530 mA·hr/g by reacting LiMn2O4 spinel compounds with n-BuLi as follows:LiMn2O4+n-BuLi→Li2Mn2O4+2n-BuLi→2Li2MnO2 The Li2MnO2 has a hexagonal close packed layered structure, similar to the structure of LiCoO2, except that the Li+ ions in Li2MnO2 occupy the tetrahedral sites instead of the octahedral sites as in LiCoO2. However, the Li2MnO2 compounds formed according to Thackeray's methods are unstable. In particular, Thackeray notes that the layered structure of Li2MnO2 is unstable and that it converts back to the spinel framework upon delithiation. This is undesirable because repeated conversion between layered and spinel structures decreases capacity retention and results in voltage gaps.
A doped lithium manganese oxide preferably exhibits a high usable reversible capacity and good cycleability to maintain reversible capacity during cycling. LiMn2O4 can generally only be operated at 115-120 mA·hr/g with good cycleability. Furthermore, Li2MnO2 compounds are expensive to make and are unstable when made according to available methods. Therefore, there is a need to produce a lithium metal oxide that exhibits an improved reversible capacity and good cycleability while maintaining thermal stability.